Parkinson's disease is a neurodegenerative disease. It is the second most common neurodegenerative disease after Alzheimer's disease and affects more than 1% of the population above the age of 65. Parkinson's disease is clinically characterised by resting tremor, bradykinesia and muscular rigidity. Pathologically, the disease is characterised by loss of dopaminergic neurons with the consequent decrease in dopamine levels in the brain and by aggregation of the protein α-synuclein in the dopaminergic neurons. These aggregations called Lewy-bodies are composed of insoluble α-synuclein phosphorylated at serine-129 and ubiquitin. Current Parkinson's disease therapeutic intervention strategies aim at increasing the dopamine levels in areas innervated by dopaminergic neurons in the brain. Levadopa is a precursor of dopamine, and it is therapeutically used to increase dopamine levels. Carbidopa is an inhibitor of the enzyme aromatic-L-amino-acid decarboxylase also known as DOPA decarboxylase, and it is often co-administered with levadopa to increase the fraction of levadopa which reaches the clinically relevant regions in the brain. Monoamine oxidase B inhibitors are administered to increase the levels of dopamine by blocking the metabolism of dopamine. As an alternative, dopamine agonists are administered to stimulate dopaminergic neurons, an effect similar to that obtained by increasing the dopamine levels. Although these therapies provide significant symptomatic benefit to the patient, they are also associated with adverse side effects and often become ineffective after prolonged treatment. Importantly, neither of the existing therapies addresses the underlying and disease causing problem, i.e. the progressive loss or inactivation of dopaminergic neurons.
Leucine-Rich Repeat Kinase 2 (LRRK2) is a 2527 amino acid protein involved in catalysing phosphorylation and GTP-GTD hydrolysis. The NCBI reference sequence for human LRKK2 mRNA is NM_198578.2. Evidence is mounting showing a relationship between LRRK2 and the pathogenesis of Parkinson's disease. It has been shown that LRRK2 phosphorylates α-synuclein at serine-129, and as discussed above this phosphorylated form constitutes a significant part of the Lewy-bodies [BiochemBiophys Res Comm., 387, 149-152, 2009]. Additionally, single nucleotide polymorphisms in functional domains of LRRK2 have been shown to cause familiar and sporadic Parkinson's disease. So far at least 6 pathogenic variants have been identified, i.e. Gly2019Ser, Ile2020Thr, Arg1441Cys, Arg1441Gly, Arg1441His and Tyr1699Cys [Parkinsonism Rel. Dis., 15, 466-467, 2009; Movement Dis., 25, 2340-2345, 2010; Neuron, 44, 601-607, 2004; and Lancet, 365, 412-415, 2005]. Importantly, the clinical features of Parkinson's disease associated with LRRK2 mutations cannot be distinguished from those featuring in idiopathic Parkinson's disease. This strongly suggests a common pathogenic mechanism and that LRKK2 activity is a rate-limiting factor in Parkinson's disease progression [FEBS Journal, 276, 6436-6444, 2009].
The most common pathogenic form of LRRK2-associated Parkinson's disease is found in carriers of the amino acid substitution Gly2019Ser in the kinase domain of the LRRK2 protein. Gly2019Ser Parkinson's disease is inherited in an autosomal dominant fashion suggesting a gain-of-function mutation of the LRRK2 protein. In support of this notion, biochemical studies have shown that both the glycine to serine substitution at amino acid position 2019 as well as isoleucine to threonine substitution at amino acid position 2020 in the kinase domain lead to an increased kinase activity of LRRK2 [Proc. Nat. Acad. Sci USA, 102, 16842-16847, 2005]. This suggests a causal involvement of overactive LRRK2 in the pathogenesis of familiar forms of Parkinson's disease. Thus, inhibitors of LRRK2, including e.g. the G2019S and I2020T mutations, could be used as disease modifying treatment in familiar Parkinson's disease.
In cellular and animal studies several phosphorylation sites in the LRRK2 protein have been identified. Most prominent, phosphorylation of LRRK2 at two conserved residues serine at amino acid position 910 and serine at amino acid position 935 in human LRRK2 located just amino terminal to the leucine-rich repeat domain mediates binding to 14-3-3 proteins. Phosphorylation at serine residues 910 and 935 were shown to be dependent on an active LRRK2 conformation and further, that LRRK2 kinase inhibitors can inhibit phosphorylation at these two sites [Biochem J., 430, 405-13, 2010; J Neurochem., 120:37-45, 2012].
LRRK2 kinase inhibitors have been shown to concentration-dependently inhibit LRRK2-Ser910 and LRRK2-Ser935 phosphorylation in cellular models expressing LRRK2 and LRRK2-G2019S as well as human LRRK2-expressing lymphoblastoid cells from PD patients homozygous for the LRRK2 G2019S mutation. In addition, LRRK2 kinase inhibition dose-dependently inhibits LRRK2-Ser910 and LRRK2-Ser935 phosphorylation in mouse brain after in vivo administration of compound. [ACS Med. Chem. Lett., DOI: 10.1021/ml300123a, 2012.].
Common single nucleotide polymorphisms of LRRK2 have also been associated with Parkinson's disease [Nat Genet. 2009 December; 41(12):1308-12] [Mov Disorder. Oct. 31, 2012; doi: 10.1002/mds.25226]. A recent genome wide association meta-analysis study where correction for G2019S carrier status was performed indicated that common LRRK2 variants with minor allele frequency (MAF) above 1% also are associated with an increased risk of Parkinson's disease [Lancet. 377, 641-649, 2011]. Further, investigations of common exonic polymorphic variants have highlighted several LRRK2 Parkinson's disease risk variants: in Caucasians the M1646T mutation, and in the Asian population the A419V mutation and also the previously found G2385R mutation [Lancet Neurol. 10, 898-908, 2011]. This indicates that LRRK2 inhibitors also could be useful as disease-modifying treatment in Parkinson's disease patients carrying common genomic LRRK2 variants such as M1646T, G2385R and A419V.
Indeed, as discussed above, as the clinical features of LRRK2 associated and idiopathic Parkinson's disease are very similar this also suggests that LRRK2 inhibitors could be useful for the treatment of sporadic PD.
As established above, LRRK2 inhibitors may by used in the treatment of Parkinson's disease and particular mention is made of Parkinson's disease associated with mutations in LRRK2, such as Gly2019Ser. Moreover, LRRK2 inhibitors are also expected to be useful in the treatment of other diseases which are associated with LRRK2. LRRK2 has been identified as a core component in Lewy bodies and is thus expected to be useful in the treatment of Lewy body dementia [Neuropathol. Appl. Neurobiol., 34, 272-283, 2008]. Expression of LRRK2 mRNA is highly enriched in brain, lungs, kidney, spleen and blood suggesting that functional impact of increased LRRK2 activity is likely to be most relevant in pathogenic and pathologic conditions associated with those regions. Support for that notion can be found in studies showing an increased risk of non-skin cancer in LRRK2 Gly2019Ser mutation carriers and especially for renal and lung cancer [Mov. Disorder, 25, 2536-2541, 2010]. Over-expression of LRRK2 by chromosomal amplification has also been identified in papillary renal and thyroid carcinomas. Also, genetic association of LRRK2 has been reported to diseases in where aberrant responses of the immune system are involved. This is the case for inflammatory bowel diseases such as Crohn's disease and ulcerative colitis as well as for leprosy [Nat Genet. 42, 1118-1125, 2010; Inflamm. Bowel. Dis. 16, 557-558, 2010; N Engl. J Med. 361, 2609-2618, 2009; Inflamm. Bowel. Dis. doi: 10.1002/ibd.21651, 2011].